To make the pastry, sift together the flour, Clabber Girl Baking Powder and salt. Set aside. Use a stand mixer or hand held beaters to cream the butter and sugar until fluffy, about 3 minutes. Add the egg until incorporated and add the milk and vanilla. Whisk in half the flour until incorporated, then mix in the rest. Pat the dough down to a 1/2 inch thickness, wrap in plastic and refrigerate. This part can be done up to one day ahead.
Set a rack in the upper third of the oven and preheat to 375 .
Line a cookie sheet with parchment or use a tart pan. Roll the pastry on a lightly floured surface to a 13 inch circle. Transfer pastry.
Mix together the flour and 2 tablespoons of the sugar and spread over the pastry leaving a 1-inch border. Arrange peaches on top of the sugar mixture, then tuck the huckleberries in the spaces around the fruit.Sprinkle with the remaining 3 tablespoons of sugar. Fold in outer edge of pastry.
Bake, lightly covered with foil, about 40 minutes. Remove the foil and bake 5 to 10 more minutes until juices bubble. Allow to cool and serve.